1. Field of the Invention
The present invention relates to an optical retro-reflection device, and more articularly to an optical retro-reflection device employing switchable holographic diffraction devices.
2. Description of the Related Art
Retro-reflective devices and materials are those that reflect and re-emit light incident on such devices and in a direction that is parallel to that of the source of the incident light. Put another way, such materials and devices reflect light directly back to the source of the light they are reflecting or re-emitting. Such materials and devices are widely used in the areas of nighttime transportation and safety. They are used to identify highway lanes and signs using the light emitted from vehicle headlights for example.
It would be advantageous to programmably control the color of the light that is reflected or re-emitted by such devices and materials for numerous applications. For example, it would be beneficial to control the color of light reflected by retro-reflector arrays that form a reconfigurable display for a highway warning or informational sign.
According to the present invention, there is disclosed an optical retro-reflection device comprising a retro-reflector which is disposed to receive light incident upon the device and to reflect and re-emit the light from the device in a direction parallel to its direction of incidence. A plurality of holographic diffraction devices are disposed to act upon the incident light between the source of the light and its being incident upon and being re-emitted by the optical retro-reflection device. Each of the holographic diffraction devices are operative to act upon a different wavelength band of radiation (e.g. red, green or blue visible light) and are switchable between an active, diffracting state and an inactive, non-diffracting state. A control circuit coupled to the holographic diffraction devices is operative to selectively switch the holographic diffraction devices between their active and inactive states.
The retro-reflector can be a single cube corner prism or three orthogonally arranged plane reflectors. Alternatively, the retro-reflector can be an array of such prisms or reflectors. In the case where the retro-reflector is an array of such devices, the optical retro-rereflection device can include a set of holographic diffraction devices each associated with one of the individual prisms/reflectors making up the array, or the diffraction devices can each be associated with a group of the prisms/reflectors. As a further alternative, the retro-reflector can be an array of Microspheres, or an array of micro-prisms each of which is in the form of a cube corner. Again, a single diffraction device can be associated with the entire array, or a number of diffraction devices may be associated each with a portion off the array.
In a preferred embodiment, the holographic diffraction devices are each composed of a transmission hologram and are disposed optically in front of the retro reflector with respect to the direction of incidence of the light. The transmission holograms preferably operate in an additive mode.
In an alternate preferred embodiment, the holographic diffraction devices are each composed of a reflection hologram, and the retro-reflector is configured as three orthogonally-arranged reflection devices each of which is composed of a filter and one of the holographic diffraction devices. The filter of each reflection device is disposed optically in front of its associated holographic diffraction device, and is operative to transmit radiation of a wavelength band which corresponds to that intended to be acted upon by that holographic diffraction device. The filter of each reflection device is an interference filter, preferably a dichroic filter. A light absorbing element is disposed on the surface of the holographic diffraction elements opposite to the surface that is coupled to the filter.
At least one of the reflection devices can deviate from the orthogonal with respect to the other two reflection devices, when further composed of an optical device that is operative to compensate for this deviation. This compensating optical device can be a Fresnel lens or a Fresnel prism, in which case it is preferable that such lens or prism be optically interposed between the respective filter and the respective holographic diffraction device.